1. Field of the Invention
This invention relates to a method for fabricating a metal member having a plurality of fine holes.
2. Description of the Prior Art
For the fabrication of a metal sheet having a multitude of fine holes at high precision in the order of micrometers, it is the usual practice to adopt etching methods which make use of photofabrication techniques.
According to the etching method, for example, a photosensitive resin layer called a photoresist is applied entirely on one side of a metal sheet substrate on which fine holes are to be formed. The resin layer is subjected to exposure to light and development according to a photographic process, thereby removing the resin layer at portions where the fine holes are to be formed, thereby forming openings in the resin layer. The metal sheet is etched with a liquid etchant through the openings to make holes corresponding to the openings. Thus, the metal sheet having a multitude of fine holes can be fabricated.
In the method for fabricating a metal sheet having a multitude of fine holes according to the etching technique, however, the degree of etching may differ depending on the uniformity in composition of the metal sheet substrate. This will result in the variation in diameter of the fine holes being made, with the attendant drawback that desired fine holes cannot be formed with high precision.
To overcome the drawback, there has been proposed a technique of fabricating a metal sheet having a multitude of fine holes using an electrodeposition method which ensures high processing precision (Japanese Patent Publication No. 58-13355).
More particularly, according to this technique, a multitude of non-conductive portions 101 used to form fine holes are formed, as shown in FIG. 7aon a conductive substrate 100 such as a stainless steel sheet to make a master 102. Then, electrodeposition such as of nickel is performed on the surface of the master 102, thereby forming on the master 102 an electrodeposition coating 104 having a hole 103 as a non-electrodeposited portion which is free of any electrodeposit at the inner periphery of the non-conductive portion 101. This is shown in FIG. 7b. The electrodeposition coating 104 is separated from the master 102 to obtain a metal sheet 106 having fine holes 105 (103) as shown in FIGS. 7c and 8.
However, this method is disadvantageous in that the non-conductive portions 101 on the master 102 are formed according to the photoresist method and are thus weak in the strength of bonding to the conductive substrate 100. For instance, when the electrodeposition coating 104 is separated from the master 102, part of the non-conductive portions 101 may also be separated from the conductive substrate 100. Whenever the separation takes place, it becomes necessary to re-fabricate the master 102. This will undesirably lead to poor productivity, thus presenting the problem of increasing production costs. Moreover, whenever the master is re-fabricated as a fresh one, the patterns of the non-conductive portions on the respective masters are minutely changed. A problem arises in that the dimensional precision of the holes of individual metal sheets vary among the sheets.
To solve the above problem, we have already proposed a novel fabrication technique as set forth in Japanese Laid-open Patent Application No. 1-105749.
According to this fabrication method shown in FIGS. 9a to 9c, a conductive substrate 200 is formed with through-holes 201 such as by electrical discharge machining as shown in FIG. 9a. The through-holes 201 are, respectively, filled with a non-conductive material 202 to provide non-conductive portions 203 with which fine holes are formed, thereby forming a master 204 as shown in FIG. 9b. The master 204 is then subjected to electrodeposition on one side thereof to form an electrodeposition film 206 having holes 205 which are each composed of a non-electrodeposited portion corresponding to individual non-conductive portion 203 as shown in FIG. 9c. Finally, the electrodeposition film 206 is separated from the master 204 to obtain a metal sheet 106 having fine holes 105 (205) as shown in FIG. 8. In FIGS. 9a to 9c, reference numeral 207 indicates an electrode and reference numeral 208 indicates a backup plate.
The fabrication method is advantageous in that the non-conductive portions 203 are fixedly deposited on the conductive substrate 200, not permitting even a part of the non-conductive portions to be separated at the time of the separation of the electrodeposition film. In addition, because the master 204 can be repeatedly used, metal sheets each having fine holes at high precision can be readily fabricated in an efficient manner.
However, the fabrication method which makes use of the above repeatedly usable type of master has the following problems.
In the fabrication method, as shown in FIG. 10, the non-conductive portion 203 is formed at the same level as the conductive substrate 200 of the master 204. The electrodeposition film 206 is formed as slightly overlapped at the periphery of the non-conductive portion 203. Accordingly, the diameter, r, of the actually formed fine hole 105 (205) is a value which is obtained by subtracting the length, x, of the overlapped electrodeposition film 206 from the diameter, d, of the through-hole 201 with which the non-conductive portion 203 is formed. More specifically, the diameter, r, of the fine hole 105 is determined depending on the overlapped length of the electrodeposition film 206. Because the overlapped length varies depending on the electrodeposition time and electrodeposition conditions, the diameters, r, of the fine holes 105 vary, depending on the variation in the overlap length, in one metal sheet or among the repeatedly fabricated metal sheets. This presents the problem that the accuracy of the fine holes lowers according to the state of the electrodeposited film, i.e. the variation in the overlapped total length.
Although the electrodeposition film 206 can be controlled to an extent by controlling electrodeposition conditions with respect to the ratio between the film-forming speed along the direction of thickness and the film-forming speed (overlap length) along the lateral direction, the ratio takes a value of approximately 1. If the thickness of the electrodeposition film (or thickness of the metal sheet 106) is increased, it should be taken into account that the overlap length also increases substantially at the same rate as the film thickness. This means that for the formation of fine holes 205 with the same diameter, the distance between adjacent fine holes has to be increased at the same time. Eventually, the thickness of the electrodeposition film undesirably places a limitation on the number of the fine holes (i.e. hole-formation density) to be formed.